Not applicable.
Not applicable.
1. Field of the Invention
This invention is directed to thermoplastic elastomer blends with certain improved properties. More specifically, it relates to partially resin-cured thermoplastic elastomer blends with improved oil resistance when blended with, for example, an ethylene vinyl acetate resin or a silicone-modified olefinic rubber. The resulting compounds have improved oil resistance over similar peroxide-cured compositions.
2. Description of the Related Art
Oil resistance is important to the wire and cable industry and end users of their products. Oil resistance may be defined as the percent retention of the original unaged tensile strength and elongation at break. It therefore follows that a higher percent retention of the original unaged tensile strength and elongation at break is desirable, as this is indicative of the material having better oil resistance.
U.S. Pat. No. 4,311,628 is directed to a blend of an olefin rubber and a polyolefin resin in which the rubber is fully cured with a phenolic resin. However, there is no mention of using an olefin vinyl acetate copolymer or a silicon modified copolymer as an additional component.
In one aspect, the present invention relates to a composition which comprises:
a) 100 parts by weight of a monoolefin copolymer or terpolymer;
b) from about 5 to about 150 parts by weight of a polyolefin resin;
c) from about 10 to about 80 parts by weight of one of the following:
i) a copolymer of an olefin with a carboxylic acid and/or an ester of a carboxylic acid; or
ii) a silicon modified olefinic rubber; and
d) from about 1 to about 10 parts by weight of a phenolic curative.
In another aspect, the present invention relates to a method which comprises curing the following composition:
a) 100 parts by weight of a monoolefin copolymer or terpolymer;
b) from about 5 to about 150 parts by weight of a polyolefin resin;
c) from about 10 to about 80 parts by weight of one of the following:
i) an olefin vinyl acetate copolymer with a vinyl acetate content of from about 15% to about 60%; or
ii) a silicon modified copolymer;
wherein the curing is performed with a phenolic curative.
The monoolefin copolymer or terpolymer component of the compositions of the present invention includes monoolefin copolymer rubbers and terpolymer rubbers. Suitable monoolefin copolymer rubbers include essentially non-crystalline, rubbery copolymers of two or more alpha-monoolefins, optionally copolymerized with at least one polyene, usually a diene. However, saturated monoolefin copolymer rubber, commonly called xe2x80x9cEPMxe2x80x9d rubber, can be used, for example copolymers of ethylene and propylene. Examples of unsaturated monoolefin copolymer rubber, commonly called xe2x80x9cEPDMxe2x80x9d rubber, which are satisfactory comprise the products from the polymerization of monomers comprising two monoolefins, generally ethylene and propylene, and a lesser quantity of non-conjugated diene. Suitable alpha monoolefins are illustrated by the formula CH2xe2x95x90CHR in which R is hydrogen or alkyl of 1-12 carbon atoms, examples of which include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 2-methyl-1-propene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3,3-dimethyl-1-butene, 2,4,4-trimethyl-1-pentene, 5-methyl-1-hexene, 1,4-ethyl-1-hexene and others. Satisfactory non-conjugated dienes include straight chain dienes such as 1,4-hexadiene, cyclic dienes such as cyclooctadiene and bridged cyclic dienes such as ethylidenenorborene. Grades of EPM and EPDM rubbers suitable for the practice of the invention are commercially available.
The polyolefin resin component of the compositions of the present invention include crystalline, high molecular weight solid products from the polymerization of one or more monoolefins by either high pressure or low pressure processes. Examples of such resins are the isotactic and syndiotactic monoolefin polymer resins, representative members of which are commercially available. Examples of satisfactory olefins are ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 2-methyl-1-propene, 3-methyl-1-pentene, 4-methyl-1-pentene, 5-methyl-1-hexene and mixtures thereof. Commercially available thermoplastic polyolefin resins, and preferably polyethylene or polypropylene, may be advantageously used in the practice of the invention, with polypropylene being preferred.
The olefin-acid/ester copolymer component of the compositions of the present invention includes copolymers of olefins with a carboxylic acid and/or unsaturated esters of carboxylic acids. Examples of satisfactory olefins are ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 2-methyl-1-propene, 3-methyl-1-pentene, 4-methyl-1-pentene, 5-methyl-1-hexene and mixtures thereof. Suitable ester monomers include vinyl acetate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, lauryl acrylate, benzyl acrylate, N,N-dimethylaminoethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, methyl fumarate, ethyl fumarate, propyl fumarate, butyl fumarate, dimethyl fumarate, methyl maleate, ethyl maleate, propyl maleate, butyl maleate, dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, and mixtures thereof. Suitable carboxylic acid monomers include the acids corresponding to the ester monomers set forth above, and mixtures thereof. Particularly preferred are copolymers of ethylene with at least about 40 weight percent vinyl acetate, preferably at least about 50 weight percent vinyl acetate.
The silicon modified olefinic rubber component of the composition of the present invention includes olefinic rubbers which have been modified with a silicon-containing polymer. Suitable olefinic rubbers include EPM and EPDM rubbers described above, as well as others including ethylene-vinyl acetate copolymer rubbers. The silicon-containing polymer component includes various organopolysiloxanes, methoxy-stopped silicones, silanol stopped silicones, etc. Each of the foregoing components may be manufactured by conventional techniques, and many are commercially available.
Particularly preferred silicon modified olefinic rubbers include silicone-modified EPDM with polydimethylsiloxane polymers with either vinyl or methyl-terminated polymer chains, preferably methyl-terminated polymer chains. The vinyl content of the polydimethylsiloxane polymer can range between 0 to 4 percent, preferably between 0.1 to 1 weight percent vinyl. The EPDM component of the silicone-modified EPDM can be 10 to 50 weight percent, preferably between 20 to 30 weight percent. Especially preferred modified rubbers are sold under the ROYALTHERM mark by Uniroyal Chemical Company. The modified rubbers may also contain additives such as fillers, etc.
The phenolic curative component of the compositions of the present invention includes any phenolic curative system which is capable of curing the monoolefin copolymer or terpolymer. A basic ingredient of such system is a phenolic curing resin made by condensation of halogen substituted phenol, C1-C10 alkyl substituted phenol or unsubstituted phenol with an aldehyde, preferably, formaldehyde, in an alkaline medium or by condensation of bifunctional phenoldialcohols. Additional information concerning phenolic curatives may be found in U.S. Pat. No. 4,311,628, the content of which is incorporated herein by reference.
It has been observed that the compositions of the present invention display improved oil resistance without the need to fully crosslink the rubber component. Thus, in a preferred embodiment of the present invention, the compositions are cured to a gel content of less than about 97%, more preferably in the range of about 85-95%, and most preferably about 90%. It will be understood that gel content refers to the weight % of curable rubber extractable in cyclohexane at 23xc2x0 C.
The compositions of the present invention may be made by conventional blending techniques which would be readily apparent to those skilled in the art. A suitable blending technique includes the use of an internal mixer, such as a Banbury. As one example, the monoolefin copolymer or terpolymer and polyolefin resin may be mixed at a temperature where the mixture starts to melt, then the other components may be added. It may be necessary to then raise the temperature in order to promote the vulcanization reaction. The mixed composition may then be discharged from the internal mixer, and further refined and cooled.
In addition to containing the components described above, the compositions of the present invention may include components conventionally used in thermoplastics, such as thermoplastic polyolefins, thermoplastic vulcanizates or conventional crosslinkable thermoset elastomers.
The compositions of the present invention are thermoplastic and may be fabricated into useful articles which would be well known to those in art, by conventional plastics processing methods, including injection molding or extrusion. Suitable products include cable insulation and jackets, electrical molded goods, weatherstripping etc.